1. Field of the Invention
The present invention relates to methods and apparatus for use in phototypesetting apparatus and more particularly to such methods and apparatus for compacting or encoding character data in one of a variety of methods that will result in the smallest quantity of character data.
2. Description of the Prior Art
In phototypesetting apparatus marketed under the trade name "METRO SET" by the assignee of the present application, photosensitive material (film or paper) is directly exposed by the light output of a cathode ray tube which is provided with a fiber optic faceplate which is the face of the tube for culminating the light output therefrom for increasing the efficiency of light transfer to the photosensitive material, minimizing the light spot size and accurately exposing the photosensitive material. The copy which is to be phototypeset is inputted into the apparatus via punched paper tape, magnetic tape, or direct computer link, which, in addition to its information content, selects the style, case, size, position (both vertically and horizontally) and slant of letters, if desired, of the type to be phototypeset. Stored in the apparatus on a magnetic disc are digital instructions for reproducing copy in up to one hundred master-type fonts of a common size. Under the control of a digital computer and in response to the input information, the scanning operation of the cathode ray tube is performed to sequentially access the coordinate points of the cathode ray tube screen with the digital instructions stored in the magnetic disc being outputted to processing circuitry for determining the light output of the cathode ray tube. The light output of the cathode ray tube as generated across the fiber optic faceplate thereof is representative of a line of type being set from left to right, with an individual letter being formed, for example, from the bottom to the top. The photosensitive material is exposed in response to the spot of light output of the cathode ray tube and is mechanically transported across the face of the fiber optic faceplate to be stored in a take-up cassette for eventual development by normal photographic techniques.
A typical format for the type in the font of mastertype consists of a matrix of 1,024 bits by 1,024 bits in a normalized em quad of 1".times.1". Thus, approximately one million bits of information are required to define the X, Y coordinate points in the format, with 0.001" being the spacial displacement of coordinate points. If bulk storage were to be employed this would require one million bits to define the coordinate points for each letter or symbol of each of the fonts. To avoid such a large storage capacity required in bulk storage, a processor is provided in the phototypesetting apparatus for compacting the digital instruction storage requirement. Thus, rather than storing individual coordinate point information, instructions in multi-bit digital word form are recorded on the font magnetic disc to which the uncompacting processor is responsive for producing the necessary outputs to control the cathode ray tube light output. A suitable uncompacting controller system and method to effect such storage and operation are disclosed in U.S. Pat. No. 4,029,947 and assigned to the same assignee as the present application.
Each of the letters and symbols in a particular type style in each of the up to one hundred fonts must be encoded in digital instruction form for recording, which is an expensive and time-consuming operation, requiring that each letter and symbol be individually analyzed so that it can be properly encoded for providing appropriate instructions to the uncompacting processor. The time expense involved is justified in the case of providing fonts composed of typical letters and symbols commonly employed in phototypesetting operations. If, however, a user desires to phototypeset specialized characters, a serious problem arises in storing and encoding such characters not ordinarily found in a standard font. For example, if a newspaper has a need to print a particular company's trademark symbol in classified advertising, it would presently be necessary that this symbol be encoded and recorded according to the compacting system of the phototypesetting apparatus. As previously mentioned, this is an expensive and time-consuming operation, as well as not possible with complex logos. Present practice is for the phototypeset copy to be developed without the specialized character, with space being left for it. The character is then manually affixed to the copy for ultimate reproduction. Thus, the user of the phototypesetting apparatus is faced with the option of acquiring and storing individualized digital instructions for the specialized character compatible with the compacting processor used or to insert manually each specialized character on the developed phototypeset copy.
A method and apparatus for scanning a character or logo and storing the resultant character data in a storage device, are described in U.S. Pat. No. 4,038,493, assigned to the assignee of this invention. There is described an apparatus including a cathode ray tube (CRT) for directing a light beam in a raster fashion across a logo which is incrementally moved past the CRT, whereby the transmissive portions of the logo transmit the light beam and the opaque portions intercept the light, the transmitted light beam being detected by a photocell. The transitions from the transmissive portions of the logo to the opaque portions are detected by a threshold detector to provide a signal indicative of such a transition. As described in the noted patent, each such transition or individual coordinant point data is converted into a 16 bit word to be stored in the storage device. However, the disadvantage of such a scheme is that each transition requires a word location within the storage device. In those instances where there is a need to store a significant number of transitions, such as reproducing a logo with a half tone characteristic, the number of transitions and words to represent that logo becomes excessive thus requiring an exceptionally large storage device.
Further, there are other schemes of encoding or compacting character data, whereby the size of the storage device required is decreased. For example, run length coding and skipping light coding produce more compact representation of the transition data points. However, such methods work well only when the genreal content of the image can be characterized in advance in order to fit such coding techniques to the particular character logo. Variable length codings do offer some improvement in the data compaction techniques of images.